chemistry 151: basic chemistry chemical equations

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Chemistry 151: Basic ChemistryChapter 4 Part I: The Chemical Reaction

Last update: 8/9/21 MAR

Chemical EquationsChemical equations are like recipes in cooking: They tell a

chemist how to make something ("products") and what you'll need to make it ("reactants")

Having balanced amounts critical in cooking: too much flour can make a cake dry, and too little flour can prevent the cake from forming. Same in chemistry!

We will learn how to create a balanced chemical equation in this chapter, and in the next section, we will explore the quantities needed to actually make the products.

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Chemical equation: An expression in which symbols and formulas are used to represent a chemical reaction.

Reactant: A substance that undergoes change in a chemical reaction; written on left side of the reaction arrow

Product: A substance that is formed in a chemical reaction; written on right side of reaction arrow

Chemical Equations

Equations depict the kind of reactants and products and their relative amounts in a reaction.

4 Al(s) + 3 O2(g) ---> 2 Al2O3(s) The numbers in the front are called

stoichiometric coefficients The letters (s), (g), (l) and (aq) are the physical states of

compounds: s = solid, g = gas, l = liquid, aq = solution in water (aqueous)

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Chemical Equations

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The Law of conservation of mass states that matter cannot be created or destroyed in any chemical reaction

The bonds between atoms in the reactants are rearranged to form new compounds, but none of the atoms disappear, and no new atoms are formed.

So: Chemical equations must be balanced, meaning the numbers and kinds of atoms must be the same on both sides of the reaction arrow.

The numbers placed in front of formulas to balance equations are called coefficients, and they multiply all the atoms in the chemical formula.

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Chemical Equations are BalancedIn a balanced chemical reaction:

• atoms are not gainedor lost.

• the number ofreactant atoms isequal to the numberof product atoms.

Page III-4a-1 / Chapter Four Part I Lecture Notes


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P4 + Br26 4 PBr3

P

PP

PBr Br

PBr Br

Br

Br Br

Br BrBr BrBr BrBr Br

PBr Br

Br

PBr Br

Br

PBr Br

Br

4 P 4 P12 Br 12 Br+ +

Balance Sheet:

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Balancing Chemical EquationsThe following four steps can be used as a guide to

balance chemical equations. Example: Sulfuric acid reacts with sodium

hydroxide to create sodium sulfate and water. Balance this chemical reaction.

Step 1: Write an unbalanced equation, using correct formulas for all reactants and products.

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Step 3: Check the equation to make sure the numbers and kinds of atoms on both sides of the equation are same.

Step 4: Make sure the coefficients are reduced to their lowest whole-number value (ok here).

Step 2: Add appropriate coefficients to balance the numbers of atoms of each element.

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Balancing Equations

_ Al(s) + _ Br2(liq) ---> _ Al2Br6(s)

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Balancing Equations

-

2 Al(s) + 3 Br2(liq) ---> Al2Br6(s)

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____C3H8(g) + _____ O2(g) -->

_____CO2(g) + _____ H2O(g)

Balancing Equations



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Balancing Equations

C3H8(g) + 5 O2(g) ----> 3 CO2(g) + 4 H2O(g)

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Balance the following: Calcium + nitrogen ! Calcium nitride

remember nitrogen is a diatomic! 3 Ca + N2 ! Ca3N2

Balancing Equations

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Balancing with Polyatomic IonsMagnesium chloride + sodium phosphate → magnesium phosphate + sodium chloride

MgCl2(aq) + Na3PO4(aq) ! NaCl(aq) + Mg3(PO4)2(s)

Leave polyatomic ions as "units", don't break up when balancing, usually balance them first before other atoms

MgCl2(aq) + 2Na3PO4(aq) ! NaCl(aq) + Mg3(PO4)2(s)

3MgCl2(aq) + 2Na3PO4(aq) ! 6NaCl(aq) + Mg3(PO4)2(s)

2 PO43- = 2 PO43-

3 Mg2+ = 3 Mg2+

6 Na+ = 6 Na+

6 Cl- = 6 Cl- Balanced! MAR

Balance the following. To save time, balance polyatomic ions as units (not individual atoms):

BaCl2 + Na3PO4 → Ba3(PO4)2 + NaCl

3 BaCl2 + 2 Na3PO4 → Ba3(PO4)2 + 6 NaCl

Balancing Equations

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Balancing Equations - HintsBalance those atoms which occur in only

one compound on each side Balance the remaining atoms Reduce coefficients to smallest whole

integers Check your answer Remember the seven diatomics! HONCl

BrIF

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Test YourselfBalance the following reactions: K(s) + H2O(l) → H2(g) + KOH(aq) 2 K(s) + 2 H2O(l) → H2(g) + 2 KOH(aq) Ba(s) + H3AsO4(aq) → H2(g) + Ba3(AsO4)2(aq) 3 Ba(s) + 2 H3AsO4(aq) → 3 H2(g) + Ba3(AsO4)2(aq) PCl5(s) + H2O(l) → H3PO4(aq) + HCl(aq) PCl5(s) + 4 H2O(l) → H3PO4(aq) + 5 HCl(aq) KClO3(s) → KCl(s) + O2(g) 2 KClO3(s) → 2 KCl(s) + 3 O2(g)

practice, practice, practice!



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Types of ReactionsMost chemical reactions can be grouped into one of these six

categories:

1. Combination A + B → AB 2. Decomposition AB → A + B 3. Single Replacement AB + C → CB + A or MY + X → MX + Y

(Metals replace metals; nonmetals replace nonmetals) 4. Combustion CxHy + O2 → CO2 + H2O 5. Acid-Base HX + MOH → MX + H2O 6. Precipitation AX + BY → AY(s) + BX

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In a combination reaction, two or more reactants form one product or simple compounds combine to form one product.

+

2 Mg(s) + O2(g) 2 MgO(s)

2 Na(s) + Cl2(g) 2 NaCl(s) SO3(g) + H2O(l) H2SO4(aq)

Combination reactions are also known as addition reactions.

Combination (Addition)

A B A B

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DecompositionIn a decomposition reaction, one substance splits into two

or more simpler substances.

2 HgO(s) 2 Hg(l) + O2(g)

2 KClO3(s) 2 KCl(s) + 3 O2(g)

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Learning Check

Classify the following reactions as combination or decomposition.

H2(g) + Br2(g) 2 HBr(l)

Al2(CO3)3(s) Al2O3(s) + 3 CO2(g)

4 Al(s) + 3 C(s) Al4C3(s)

Combination

Decomposition

Combination

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Single Replacement Reactions

Single replacement reactions:

A + BC → AC + B X + BY → BX + Y

Metal (A and B) replace metals; Non-metals (X and Y) replace non-metals

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Single ReplacementIn a single replacement reaction, one element takes the

place of a different element in a reacting compound.

Metals (Zn and Ag) replacing metals:

Zn(s) + 2 AgCl(aq) ZnCl2(aq) + 2 Ag(s)

Nonmetals (Br and Cl) replacing nonmetals:

Br2(g) + 2 NaCl(aq) 2 NaBr(aq) + Cl2(g)



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Learning CheckComplete and balance the following single replacement equation:

Metals replace metals: zinc + silver nitrate →

Zn + 2 AgNO3 → Zn(NO3)2 + 2 Ag

Non-metals replace non-metals: chlorine + sodium iodide →

Cl2 + 2 NaI → 2 NaCl + I2

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Combustion ReactionsIn a combustion reaction, a hydrocarbon (containing C, H and/or O)

reacts with oxygen (O2) to make carbon dioxide and water. These are very common in organic chemistry (and in your combustion gasoline car!)

C2H4(g) + 3 O2(g) → 2 H2O(g) + 2 CO2(g)

C6H12(g) + 9 O2(g) → 6 CO2(g) + 6 H2O(g)

2 C2H4O(g) + 5 O2(g) → 4 CO2(g) + 4 H2O(g)

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Acid-Base Reactions

In an acid-base reaction, an acid (H listed first) reacts with a base (metal hydroxide) to create water and a "salt"

HNO3(aq) + NaOH(aq) H2O(l) + NaNO3(aq)

Acid-Base reactions are also a type of double displacement or exchange reactions

acid base water salt

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When equal amounts (moles) of acids (H+) and bases (OH-) are mixed together, both acidic and basic properties disappear because of a neutralization reaction. The neutralization reaction produces water and a salt.

Example: HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq) acid base water salt

Acid-Base Reactions

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acid base water salt

Example: HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq)

Acid-Base Reactions

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Precipitation ReactionsIn a precipitation reaction, reactants exchange cations, and

at least one of the products is a solid (a precipitate)

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

Precipitation reactions are also a type of double displacement or exchange reactions

precipitate



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Solubility: The amount of a compound that will dissolve in a given amount of solvent at a given temperature. When solubility exceeded, precipitates form

Pb(NO3)2(aq) + 2 KI(aq) → PbI2(s) + 2 KNO3(aq)

Precipitation Reactions

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Test YourselfBalance and classify the following reactions: C5H12(l) + O2(g) → C5H12(l) + 8 O2(g) → 6 H2O(g) + 5 CO2(g) combustion HCl(aq) + Pb(NO3)2(aq) → PbCl2(s) is a product 2 HCl(aq) + Pb(NO3)2(aq) → PbCl2(s) + 2 HNO3(aq) precipitation HI(aq) + LiOH(aq) → HI(aq) + LiOH(aq) → H2O(l) + LiI(aq) acid-base

practice, practice, practice!

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End of Chapter 4 Part I



chemistry 151: basic chemistry chemical equations

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